1. Field of the Invention
The present invention relates to the inspection of wafer boats utilized in a semiconductor wafer fabrication facility, and more specifically to an automatic apparatus for determining, in a non-contact fashion using optical sensors, whether certain mechanical dimensions of a wafer boat are within specification.
2. Description of Related Art
Slotted mass transfer wafer boats are used in semiconductor manufacturing for holding wafers during high temperature operations, such as an anneal operation performed in a furnace tube. These wafer boats are commonly constructed of a high-temperature quartz glass (hence the name "quartzware"), although polysilicon and silicon carbide materials are sometimes also used.
FIG. 1 shows a typical quartz wafer boat 100 which includes two bottom rods 104 and 106, and two side rods 102 and 108, each with a plurality of slots 110 on the inward side of each of the rods. The locations and size of the slots are such that a group of wafers may be positioned vertically in a horizontally positioned wafer boat and supported by their edges in the respective slots of the side and bottom rods.
To support a wafer, four slots are aligned in the same vertical plane. That is, a single slot from each of the two side rods and each of the two bottom rods define a single plane. The width of each slot is large enough to allow for clearance to easily insert a wafer into the slot, but should not be so large as to allow unnecessary lateral movement of the wafer once inserted. Excessive lateral movement of wafers dislodges contaminant particles that reduce the yields of wafers processed in the wafer boat. Furthermore, the slot-to-slot spacing along a given rod of the wafer boat is constant to permit the use of robotic loaders and unloaders which are programmed to adhere to a given wafer spacing. Wafer chipping and breakage can result if wafer boats are used that do not meet these specifications.
Even if a wafer boat is initially within specification, repetitive use can result in the wafer boat becoming out of specification. Repetitive heat cycling received by the wafer boat can cause sagging or warping of the horizontal rods. Additionally, cleaning operations typically remove a certain portion of the quartz material, resulting in slot enlargement over time.
Non-contact measurement systems do exist which are capable of measuring the slot width and slot locations in each of the rods. However, while very precise, most are general purpose optical measurement systems not well suited to inexpensively inspecting a wafer boat. Specifically, the measurement is slow and may involve many different equipment setups and a skilled operator, and the equipment itself is very expensive. Such equipment is more suitable for measuring calibration units, or so-called "golden units" due to the length of time needed to perform a measurement, as well as the cost. An example of this kind of measurement system is the Series 3000 Non-Contact Measurement System from Pacific Precision Laboratories, Inc.
Hence, the need remains in wafer fabrication facilities for an inexpensive apparatus that automatically and quickly determines whether wafer boats remain in specification after use, with a minimum of specialized operator training.